1. Field of the Invention
The present invention generally relates to a glass antenna system for vehicles, particularly to a glass antenna system for receiving AM and FM broadcasts.
2. Description of the Prior Art
A rod antenna has been provided as an antenna system for receiving AM and FM broadcasts. An antenna element of the rod antenna is protruded from the body of a vehicle, so that the antenna element has a tendency to be injured. As an alternative to the rod antenna, a glass antenna system has been provided in which the pattern of electrically conductive lines of the paste including silver powders (referred to as silver printed lines) are printed on a window glass of a vehicle.
In the glass antenna system using silver printed lines, an antenna pattern becomes complicated in order to realize a good sensitivity for FM band (76-90 MHz) in the case that the antenna pattern is formed on a small window such as a side window, resulting in a bad appearance of the side window.
A slot antenna has also provided which is formed in a space between an electrically conductive transparent film for reflecting sunlight provided on a glass window of a vehicle and a body of the vehicle. This kind of a slot antenna has been disclosed in Japanese Patent Publication Nos. 6-45817, 9-175166 and 2002-290145.
An antenna disclosed in Japanese Patent Publication No. 6-45817 is a slot antenna formed on a windshield of a vehicle. The technical word “slot antenna” means an antenna comprising a rectangular opening having a fixed length and width in an electrically conductive plate. The slot antenna, therefore, has both ends in a rectangular opening independently of the shape of the opening. The slot antenna disclosed in the Japanese Patent Publication is structured by a complete loop-shaped opening, so that the slot antenna does not have both ends thereof. In this meaning, this slot antenna is a slot antenna in a broad sense. This slot antenna is also has a complicated structure because an interface region is required to match the impedance of the slot antenna to that of a coaxial feeder.
The antenna disclosed in Japanese Patent Publication No. 9-175166 has a composite structure consisting of a slot antenna and an antenna element provided in the slot antenna in order to receive a wide frequency band.
A slot antenna for receiving FM and TV bands is disclosed in Japanese Patent Publication No. 2002-290145. This slot antenna is formed between an electrically conductive film and a body of a vehicle, and comprises two electrically conductive portion each for electrically connecting the electrically conductive film and the body of a vehicle to each other. In this slot antenna, the bands capable of receiving are limited to only FM and TV bands, and then AM band may not be received.